The present invention relates to an angular adjustment mechanism, surgical alignment guide and surgical instrument assembly. The present invention is particularly applied to orthopaedic surgery and especially knee surgery.
In orthopaedic knee surgery, a cut may be made to the femoral head in order to correct varus or valvus alignment. A cutting guide is used to locate the cut accurately. A cutting guide is fixed to the bone using pins and provides a stable surface to guide resection of the femoral head.
In order to ensure the cutting guide is placed correctly on the femur, an alignment system is typically used. An intramedullary rod is inserted into the intermedullary canal of the femur, providing a stable reference to the intramedullary axis of the femur. An alignment guide is disposed on this rod. The alignment guide contains a scale indicating the desired angle of the cut relative to the intramedullary axis of the femur and an angular adjustment mechanism. The cutting guide is attached to the alignment guide and advanced along the rod until it is in contact with the femur. It is aligned at the desired angle by the alignment guide. It can then be secured in place.
WO-A-2009/037471 describes an example of such an alignment guide. The instrument comprises an angular adjustment mechanism which includes an adjustment member which is rotatable about the longitudinal axis of an intramedullary rod. The adjustment member has an end surface defining a plurality of slots located between recesses or notches, giving the end of the adjustment member a castellated appearance. Each slot defines a different angle relative to the longitudinal axis. A selected slot engages a rib formed on a pivoting member, thereby rotating the pivoting member to a desired angle.
The mechanism of WO-A-2009/037471 has disadvantages because the space available for the adjustment member limits its size. The size sets a limit on the number of different slots the adjustment member can define to ensure that the pivoting member is held securely. In practice, the recess needs to be of a minimum width to engage the ridge firmly. The walls defining the recess also need to have a minimum size to prevent their deformation or failure. The limit on the number of slots limits the range of adjustment possible. For example, WO-A-2009/037471 discusses an embodiment in which the angular adjustment is in increments of 2 degrees or greater, for example, different slots define 3, 5 or 7 degrees of angular adjustment. It would be desirable to provide an alignment guide in which the angular adjustment mechanism can provide finer degrees of angular adjustment.
Accordingly, the present invention provides an angular adjustment mechanism in which facets on the outside of the adjustment member engage corresponding surfaces in a recess on a pivoting member. The use of facets allows a secure connection to be achieved without requiring intermediate walls or edges as are required using a slot on the angular adjustment member as taught by WO-A-2009/037471 (because the walls are necessary to define the recess). This allows a greater degree of angular adjustment and a further benefit of simpler operation.
According to a first aspect of the present invention there is provided an angular adjustment mechanism for a surgical instrument comprising an adjustment member configured for rotation about a longitudinal axis, the adjustment member comprising a plurality of pairs of facets arranged about the longitudinal axis; wherein each of the plurality of pairs of facets defines a respective angled axis at an angle relative to the longitudinal axis; and a pivoting member arranged to pivot about a pivot axis perpendicular to the longitudinal axis and comprising a recess for receiving the adjustment member and engaging one pair of the plurality of pairs of facets.